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December 30, 2008

Yellowstone Experiences Rash Of Small Earthquakes

by Sam Savage

Several small earthquakes jostled Yellowstone National Park for a third straight day Monday, and scientists watched closely to see whether the more than 250 tremors were a sign of something bigger to come.

Robert Smith, a professor of geophysics at the University of Utah, said swarms of small earthquakes happen frequently in Yellowstone, but it's very unusual for so many earthquakes to happen over several days.

"They're certainly not normal. We haven't had earthquakes in this energy or extent in many years," said Smith, who directs the Yellowstone Seismic Network, which operates seismic stations around the park.

He said the quakes have ranged in strength from barely detectable to one of magnitude 3.8 that happened Saturday. A magnitude 4 quake is capable of producing moderate damage.

"This is an active volcanic and tectonic area, and these are the kinds of things we have to pay attention to," Smith said. "We might be seeing something precursory.

He said they're unsure as to whether it could develop into a bigger fault or something related to hydrothermal activity. "That's what we're there to do, to monitor it for public safety."

On Monday, the strongest of dozens of tremors was a magnitude 3.3 quake shortly after noon. All the quakes were centered beneath the northwest end of Yellowstone Lake.

Park spokeswoman Stacy Vallie said a park ranger based at the north end of the lake reported feeling nine quakes over a 24-hour period over the weekend. No damage was reported, however.

"There doesn't seem to be anything to be alarmed about," Vallie said.

It's difficult to say what might be causing the tremors, Smith said. He pointed out that Yellowstone is the caldera of a volcano that last erupted 70,000 years ago.

Yellowstone remains very geologically active - and its famous geysers and hot springs are a reminder that a pool of magma still exists five to 10 miles underground.

"That's just the surface manifestation of the enormous amount of heat that's being released through the system," he said.

In recent decades, Yellowstone has had significant earthquakes as well as minor ones. In 1959, a magnitude 7.5 quake near Hebgen Lake just west of the park triggered a landslide that killed 28 people.